[oneTBB] Add task_scheduler_handle

[alexey-katranov]

Add task_scheduler_handle description (preview implementation description: https://docs.oneapi.io/versions/latest/onetbb/reference/blocking_terminate.html). There are some changes releated to get (removed) and release (became a member function) methods.

Additionally, deprecated task_arena::attach and introduce oneapi::tbb::attach.

Notify: @kboyarinov @anton-potapov

[alexey-katranov]

@rscohn2 , can you please advise about CI failure? It references a file that is added to the index page:

Warning, treated as error:
/home/runner/work/oneAPI-spec/oneAPI-spec/source/elements/oneTBB/source/deprecated/task_arena_attach_tag.rst:document isn't included in any toctree

However, it is referenced in index page oneTBB/source/index.rst:39-43:

   .. toctree::
   :maxdepth: 2
   :caption: oneTBB Deprecated Interfaces:

   deprecated/task_arena_attach_tag.rst

[rscohn2]

When you build the standalone doc, it uses index.rst, but when you build the full spec, it uses nested-index.rst.
index.rst has multiple TOC's at the same level, which is not OK when the page is nested in a parent TOC.

To fix the problem add the page to nested-index.rst